Systems Thinking for Sustainable Manure Management
North Carolina State University
Sustainability is a popular buzzword these days, but what does it really mean to be sustainable? That often depends on who you ask, particularly when it comes to the food animal industry. A livestock producer might think of maintaining economic viability in their farming operation, while their downwind neighbor may feel it is more about having fresh air to breathe and clean water to drink. Meanwhile, animal protein consumers generally focus on price, although their perceptions of quality or animal welfare could also impact purchasing choices. We can think of sustainability as a three-legged table where one leg represents the economy, one leg represents the environment and one leg represents society. If any of these legs fails – or alternatively, outsizes the others – the table falls over, unable to sustain its balance.
In today’s complex world the relationships between society, economy and environment are so intertwined that a change in any of these three aspects affects the others, often in ways we did not intend. The consequences can be challenging to undo. Consider modern pork production in the Carolinas. A hog farm today looks very different from those of a century ago. Then a farm might have a few dozen pigs kept in open lots or pasture. Nowadays, a typical finishing farm can house thousands of animals, sheltered in barns. In temperate climates, these barns are often lined-up along an earthen pond known as an anaerobic lagoon and surrounded by a patchwork of hayfields or crops. The manure produced by the pigs is flushed from the barns to the lagoon, which serves as both storage and treatment, much the same way human waste is flushed from a house and treated in a septic tank but at a much larger scale. This treatment process, known as anaerobic digestion, puts nature to work, using microbial activity to breakdown the organics in manure to plant-available nutrients, which are then applied to the surrounding fields as fertilizer.
It is an efficient system for both pork production and manure management and was initially believed to be sustainable as well. After all, manure has been recycled by humans as fertilizer for centuries to promote soil health and increase crop yields. Over the past few decades however, as animal production facilities have improved efficiency through concentrated expansion, the feed required by those animals exceeds the amount of feed produced locally. At the same time, the manure produced by those animals exceeds the needs of crops that are grown in the same region. Thus, feed nutrients must be imported but only half of those nutrients are exported again as meat, while the rest remains as manure. Although economical for protein production, this reality neglects the buildup of excess nutrients in the local system. That excess can lead to unintended consequences such as odor, greenhouse gas emissions, water pollution from seepage or runoff and negative effects on community health and quality of life.
These are the unintended consequences of too much growth in the economic leg of our sustainability table while ignoring the environmental and social impacts.
The solution seems simple – just move the manure from regions of overproduction to areas that can use it effectively, closing the nutrient loop. The difficulty is that with the current lagoon system, swine manure is diluted and handled as a liquid, which – while practical for on-farm management – makes it heavy and expensive to transport off-farm relative to its nutrient value. Dewatering is needed to reduce that volume-to-weight ratio, but most separation technologies are energy-intensive and costly processes that typically generate multiple waste streams that must then be handled separately. A promising alternative is to use a common desalination technology adapted to distill liquid manure under a vacuum. The vacuum reduces the boiling temperature of water, reducing the required energy input. This process would yield usable water through the distillation process, plus an organic fertilizer concentrate, and has been used successfully on the liquid digestate produced in some European biogas plants. For this technology to be useful for swine farms it must be technically and economically feasible.
In my research as a FFAR Fellow I am conducting bench-scale studies to characterize the water distillate and the manure concentrate that are produced using this process on different treatments of flushed swine manure, including anaerobic digestion and acidification. These studies will help identify how feedstock composition affects evaporation rates and how well nutrients are conserved in the manure concentrate. From this data we can estimate how different combinations of treatments or additional processing could yield a variety of outputs such as potable water for livestock, organic fertilizers in liquid or pelletized forms, or individual nutrients such as ammonium sulfate. I will engage with swine producers in the design process to identify which potential application strategies meet their needs. Using this and other expert input to develop a conceptual system model, we can evaluate how the proposed dewatering technology might assimilate into the existing infrastructure that supports the current lagoon-sprayfield system, and what combination of community support, policy incentives and potential markets for manure-based products are needed for implementation and adoption.
The goal of this systems thinking approach is to demonstrate how bringing multiple perspectives to the “sustainability table” when designing technology can provide balanced support to practical solutions for complex agricultural systems. This work was made possible through joint funding from the Foundation for Food & Agriculture Research and Waste 2 Green, LLC of Cocoa, Florida. This financial support and the professional skills I have developed as a member of the 2018-2021 cohort of FFAR Fellows have provided me opportunities to not only engage in meaningful research, but also to share that research with stakeholders from industry to policy whose common interests include sustainable agriculture.